1. Field of the Invention
The present invention relates to a contact mechanism in an electromagnetic switchgear having fixed contacts and a moving contact.
FIG. 1a is an overall sectional view of a conventional electromagnetic switchgear.
An electromagnetic switchgear 1 of the sort shown in FIG. 1a has a magnetic path casing 2, and an exciting coil 3 which is wound on a plastic bobbin 4. The exciting coil 3 consists of an absorbing coil 3a of thick conductor material and a holding coil 3b of thin conductor material. A spool 31 for winding the coils is provided at the rear end of the bobbin 4; the lead conductor 30a of the absorbing coil 3a and the lead conductor 30b of the holding coil 3b are led out therefrom, respectively.
Further, the electromagnetic switchgear 1 further has a fixed iron core 5, a guide bushing 6 made of non-magnetic material, the guide bushing 6 being fitted to the inner periphery of the bobbin 4 and secured to the fixed iron core 5, a moving iron core 7 opposite to the fixed iron core 5, the moving iron core 7 being supported in such a way that it is slidable on the inner periphery of the guide bushing 6 in the axial direction, a hook 8 with its rear end portion inserted into a hole 7a bored in the moving iron core 7, and a shift lever 9 for moving a start overrunning clutch (not shown), the shift lever 9 being coupled to the hook 8. Further, the electromagnetic switchgear 1 has a spring receptacle 10 secured to the end of the hole 7a, a buffer spring 11 for pressing the hook 8 in the backward direction, a reset spring 12 for advancing and resetting the moving iron core 7, and a resin-molded cap 13 coupled via a rubber packing 14 to the end of the fixed iron core 5 and caulked at the end of the magnetic path casing 2.
Moreover, the electromagnetic switchgear 1 has a pair of fixed contacts 15a and 15b (only one of which is shown), their terminal bolts 16a and 16b being passed through the cap 13 outwardly and fixed. The terminal of a lead wire from a power supply (storage battery) is connected to one terminal bolt 16, whereas the terminal of a lead wire for the brush of an electric motor is connected to the other terminal bolt 16b. In addition, the electromagnetic switchgear 1 has a moving contact 20 opposite to the pair of fixed contacts 15a and 15b and supported with a moving rod 21 via an insulating member 22, a retaining ring 23, a compression spring 24 for giving the moving contact 20 a contact pressure, a compression spring 15 for advancing and resetting the moving rod 21, and an external terminal 32 connected to the lead conductors 30a and 30b of the coils with solder 33, the external terminal 32 also being connected to a lead wire from a start switch of an internal combustion engine.
The operation of the electromagnetic switchgear will subsequently be described. When the start switch (not shown) is actuated, the absorbing coil 3a and the holding coil 3b are caused to conduct and the moving iron core 7 is attracted to the fixed iron core 5 and thereby moves back. The hook 8 also moves back to rotate the shift lever 9, thus advancing the overrunning clutch (not shown). On the other hand, the moving contact 20 that is geared to the backward movement of the moving iron core 7 is moved back to close the fixed contacts 15a, 15b. Power is thus supplied to the circuit of the electric motor so as to rotate its armature.
A description will then be given of the prior art mechanism of the contact between the fixed contacts and the moving contact.
FIG. 1b is an elevational view of the state in which the fixed contacts are kept in contact with the moving contact in the electromagnetic switchgear of FIG. 1a. In this example, the fixed contacts 15a, 15b and the moving contact 20 are generally made of copper or the like.
In the prior art example shown in FIGS. 2a and 2b, moreover, a contact member 50 of Ag, which is substantially circular, wear resistant and excellent in electric characteristics has been fitted to the whole head area of the fixed contact 15a in order to increase its wear resistance.
The contact mechanism of the conventional electromagnetic switchgear having the fixed contacts and the moving contact has the following problems.
In the case of the electromagnetic switchgear shown: in FIGS. 1a and 1b, the portions in which the moving contact abuts against the fixed contacts are liable to wear and this makes these contact unfit for long-term use.
In the case of what is shown in FIGS. 2a and 2b, on the other hand, as the contact member 50 of Ag having excellent electric characteristics has been fitted to the substantially whole area of each fixed contact 15a, the switchgear of this sort tends to become costly and therefore can hardly be mass-produced.
The wear of the fixed and moving contacts, if analyzed, is caused in such a state that, as shown in FIGS. 3a and 3b, the metal at the contact into which electrons flow (against which electrons hit) melts and becomes stuck on the opposite contact. Incidentally, a circuit arrangement of the electromagnetic switchgear is shown in FIG. 4.
In the meantime, the contact between the fixed contacts and the moving contact is not determined by a mutual projected area and the contact therebetween remains partial, depending on the surface tolerance and inclination of the contact surface. The partial abutment expedites the fusion of the metal, thus causing the wear of the contact.